1. Field of the Invention
The present invention relates to an undesired radiation suppression device which causes the decay of a harmful and unnecessary magnetic field that is irradiated externally from a deflection yoke of a cathode ray tube in a display monitor or the like, and more particularly to a radiation suppression device that is provided with an erasing coil for extinguishing a leak magnetic field.
2. Description of the Prior Art
In general, the regulated undesired radiation frequency which is demanded by the VDE (Verband Deutscher Elektrotechniker) standard in West Germany shall be over 10 KHz and it has been an unsolved problem for a cathode ray tube display unit to deal with, inter alia, unnecessary radiation that is produced by a horizontal deflection current.
The production of the unnecessary radiation results from a leak magnetic field generated by a deflecting coil through which the horizontal deflection current flows, a flyback transformer, a horizontal separation control coil and a horizontal linearity control coil. The unnecessary radiation quantity that is irradiated outward consists of a certain proportion amounting to about 70 percent in the deflection yoke, about 20 percent in the flyback transformer and about 10 percent in other horizontal separation control coils and the like.
FIG. 1a is a side view of a deflection yoke 1 showing only a core 1a and a horizontal deflecting coil 1b. FIG. 1b is a front view of the horizontal deflecting coil 1b observed from the front side, that is, from the side of the funnel of a cathode ray tube 2. When the horizontal deflection current flows through the horizontal deflecting coil 1b, a magnetic field is produced instantaneously in the direction shown by a dotted line arrow. In this case, the horizontal deflection magnetic field 6c is produced in the vertical direction in the space surrounded by the core 1a and at the same time a separate leak magnetic field 6a, 6b is produced at the crossover coil portion 1c, 1d before and after the horizontal deflecting coil 1b which is to be irradiated in the external space (FIG. 1c).
FIG. 2 is a partly cut-away perspective view showing a prior art cathode ray tube provided with radiation suppression device. The source of undesired radiation such as the deflection yoke 1 mounted onto the cathode ray tube, the flyback transformer (not shown) and the like is surrounded by a chassis 3, a radiation plate 4 and a cover plate 5 that are composed of a metal plate with good electrical conductivity or a magnetic material, and the leak magnetic field is confined to the inner portion to reduce the level of undesired radiation irradiated outwardly to the external portion.
Since the prior art radiation suppression device has a structure in which the source of undesired radiation is surrounded by the above-described magnetic shield plate, the display monitoring apparatus is not only restricted to a considerable extent from the view point of structure, but there are also problems in that the efficiency of heat radiation of the apparatus is low and the apparatus is high in cost.